Listen to this article

The white stucco townhouses of Cornwall Terrace overlooking Regent’s Park in London were designed by Decimus Burton, the 19th century English architect, garden designer and protégé of John Nash. At more than 10,000 sq ft each, these splendid examples of Regency architecture could not in any way be described as mean or cramped. Nonetheless, Westminster council is considering a planning application from a member of the Qatari royal family, who owns 1, 2 and 3 Cornwall Terrace, to combine the properties into a single home which, at 34,155 sq ft, would be more than 30 times the size of the average UK house.

If this scheme was a one-off it could be ignored as the whim of a wealthy eccentric. Yet in north London workmen are also renovating and expanding Witanhurst, a historic property in Highgate, that, when completed, will be the city’s second largest private house after Buckingham Palace at 90,000 sq ft.

Meanwhile, developer Christian Candy is extending Gordon House in Chelsea into a 25,000 sq ft property; and the Reuben Brothers are converting the Grade I-listed, former In & Out Club building in Piccadilly into a 53,426 sq ft home, to go on sale for £250m.

There is no formal definition for what constitutes a superhouse, though it is widely accepted to be a property which lies within prime central London — or its high-end suburbs — which measures at least 20,000 sq ft.

Prices are dependent on size and location but they will be in the tens of millions. And they are a rare breed: Savills estimates there are only 1,500 homes in the £20m-plus price bracket in the UK.

Buyers for superhomes tend to be from overseas. David Adams, managing director of John Taylor estate agents, says they are often trophy hunters from Russia and the Middle East. Whether they intend these houses to be homes or investments is up for debate. “It may be that their financial advisers have told them to park say 3 per cent of their net worth in London and perhaps they think a single house is easier to manage than several flats,” he says.

Adams has on his books a rundown Grade II-listed house in Kensington measuring 12,000 sq ft and priced at about £60m. The house comes with planning permission to be extended to about 30,000 sq ft. This project, Adams estimates, would cost £20m, but the completed house could be worth anything from £90m to £110m.

Still, finding a buyer for a superhouse can be tricky. Trevor Abrahmsohn, managing director of Glentree International, is selling Heath Hall, a 27,000 sq ft house on Bishops Avenue in Hampstead, north London. The property first went on sale in August 2012 with a price tag of £100m, subsequently cut to £65m. Today Abrahmsohn says offers above £30m would be considered. “They are not easy to sell,” he concedes. “They need placing with one of the richest families in the world and it is a delicate process which does take time.”

Globally, London’s monster houses appear to be something of an anomaly. Kirk Henckels, vice-chairman of Stribling estate agents, says that in New York a house of 10,000 sq ft or more would be considered exceptionally large. Henckels feels there is little appetite in the city for supersized homes. “We are starting to see [developers] creating larger spaces downtown, but I think the entire market is seriously overestimating global wealth,” he says. In space-constrained Hong Kong Simon Smith, a senior director of Savills, says it is unusual to find a house of more than 5,000 sq ft.

Many of the buyers of London’s largest houses are from former Soviet bloc nations where prime homes are smaller. Elena Yurgeneva, of Knight Frank, says in Moscow the vast majority of properties in the prime centre of the city are apartments. There are big houses in some suburbs, notably Serebryany Bor, in the northwest, but few exceed 20,000 sq ft. Buyers seeking really huge homes must head to the Rublyovskoye highway. Alexander Shatalov, chief executive and partner of IntermarkSavills, highlighted the recent sale of a 67,813 sq ft mansion (with 2.5 acres) in a gated community in Moscow, for $25m.

If approved, it will be more than 30 times the size of the average UK house

In Paris, the largest private house that Laurent Demeure, from Hamptons International, is aware of is a 20,000 sq ft mansion belonging to the actor Gérard Depardieu. It was put on sale in 2012 for €50m when Depardieu quit France in protest at its high tax regime. By the end of 2012 its price had been reduced to €35m. The property has subsequently been taken off the market.

While London appears to be the world’s superhome hotspot, there are signs of change ahead. On December 2 Kensington and Chelsea council learned that its bid to limit the size of basement extensions had been approved by a government planning inspectorate. This is likely to mean the end for basements of more than a single storey, with other councils, inevitably, set to follow suit.

A day later George Osborne, the chancellor, announced changes in stamp duty which will mean significant increases for buyers of multimillion-pound homes. Previously buyers paid a 7 per cent tax on homes above £2m. The new rules see a more complex system of taxation but crucially the tax will rise to 12 per cent on the portion of the price paid above £1.5m.

There is also the looming spectre of the Labour party’s plans for a mansion tax on homes worth £2m or more, and the Liberal Democrats’ proposal for higher rates of council tax on more expensive homes.

“Mansion tax could kill off these big houses,” says Adams. “Maybe they will start buying big houses and converting them into lateral flats instead, each in the name of a different family member.”

There are too few sales each year to provide accurate price data on superhomes, but prices in London’s £10m-plus bracket have fallen 1.3 per cent in the year to September, according to Savills.

Lucian Cook, director of residential research at Savills, sees the slight drop as a natural correction following strong price growth between March 2009 and June 2012. During this period the average price of £10m-plus homes rose 80 per cent to stand at 46 per cent above the pre-recession peak.

The increase in stamp duty on homes priced at above £2m may also have played a part. Over the next few months, Cook thinks the £10m-plus market will stagnate—“and I would not rule out modest falls”—as buyers hold off until they know the result of next year’s general election.